1. Field of Invention
This invention is in the field of locating scatterers generating radar returns presented within a Synthetic Aperture Radar image.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform.
The plurality of returns creating a SAR image generated by the transmitted pulses along a known path of the platform make up a frame length and is descriptive of a scene. During the frame length, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image descriptive of a scene is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the frame length.
The plurality of pulses transmitted during an SAR frame length, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the “length” traveled by the antenna during the frame length. The clarity of a SAR image is dependent on the quality of the motion compensation applied to each radar return prior to SAR image computation. The SAR process depends on the coherent, phase accurate summing of radar returns. Slight deviations in the accuracy of the motion compensation of incoming pulses introduces distortions over the whole SAR image, reducing its utility.
The Range Migration Algorithm (RMA), a method based on convolution, is a popular way to focus the radar returns forming a SAR image, assuming straight line motion over a flat earth. Inaccuracies arise with the RMA method because the earth is not flat, being in fact ellipsoidal. Some of these inaccuracies are corrected in an ad hoc fashion, but become less accurate as the range and squint angle become larger. One parent application describes a method for compensating radar returns from a spherical, non-planar scene acquired with a platform moving in a non-linear track, the Conformal Range Migration Algorithm (CRMA).
One aspect of SAR mapping using above methods is the need to identify accurately the ground location of the target presented within the SAR image. In the prior art, ground target location was approximated using heuristic methods, but errors in positioning targets within the SAR image arose. It is the minimization of target positioning errors within the SAR image that this invention strives to achieve.